1. Field of the invention
The present invention relates to the improvement of an air flow meter for internal combustion engines and a manufacturing method thereof, and particularly to an improved air flow meter having a structural configuration which is suited for manufacture with a reduced manufacturing cost.
2. Description of the related art
As is well known, an internal combustion engine is equipped with an air flow meter for metering the amount of intake air supplied to the engine with a view toward attaining exhaust gas purification, economic fuel consumption and so on. Such an air flow meter is usually located in an appropriate portion of the suction system of the engine between the air cleaner and the throttle valve.
As a typical example, an air flow meter of this type is disclosed, for example, in the Nishimura et al U.S. Pat. No. 4,709,581, which is assigned to the same assignee as the present application. The Nishimura et al air flow meter comprises a body member, in which an air path having a certain bore is defined, and a metering element, such as a known heat wire sensor, which is located in an appropriate portion within the defined air path. The body member is, as mentioned above, equipped between an air cleaner and a throttle valve, so that the metering element meters the amount of intake air sucked into the internal combustion engine.
It is also known that an air flow meter is generally designed to provide the most suitable metering range of air flow rate from the point of view of metering accuracy, as well as the reliability and durability, of the air flow meter; i.e., the lower limit of the metering range is determined based on the required metering accuracy and the upper limit thereof depends on the expected reliability and durability of the flow meter.
On the other hand, the required amount of intake air to be supplied to an internal combustion engine differs to a large extent, depending on the stroke volume of the engine. If, therefore, the bore of an air path of an air flow meter is always constant irrespective of the stroke volume of the engine, the flow rate of the intake air flowing near the metering element of the flow meter will vary widely in accordance with the stroke volume of the engine. The change in the flow rate of the intake air may consequently exceed the lower or the upper limit of the aforesaid suitable metering range.
To prevent this, there have been produced several varieties of air flow meters for engines having different stroke volumes. In such air flow meters, an air path defined in the body member has a different bore in accordance with the stroke volume of the engine, to which the air flow meter is to be equipped, whereby the change in the flow rate of intake air flowing near the metering element is established within a suitable range as mentioned above. By way of example, the assignee of the present application has manufactured three or four varieties of air flow meters having bores between 60 mm and 80 mm for engines having a stroke volume of from one to five liters.
In such a case, however, a number of kinds of parts are required for assembling several varieties of air flow meters, so that the following inconvenience results. Firstly, the manufacturing cost of parts for air flow meters cannot be reduced sufficiently, because many kinds of parts to be used in relatively small amounts must be prepared. As a result, the total cost of manufacture of such an air flow meter could not be reduced sufficiently.
Further, there is another problem in the manufacturing process. Namely, since may kinds of parts are required for several varieties of air flow meters, assemblers are forced to pay careful attention to the correct selection of parts to be fitted for each type of meter and the proper assembly thereof. Consequently, there the good possibility to cause manufacturing errors as a result of false selection of parts and improper assembly of parts.